A polarizing plate protecting film is used for protecting a polarizing film. The polarizing plate protecting film is provided on both sides of the polarizing film to form a polarizing plate. Hitherto, a displaying quality of a liquid crystal display has been improved by an optical compensation film which is a viewing angle compensation film having an optical retardation film. Recently, function of an optical retardation film has been added to a polarizing plate protecting film. As a result, the polarizing plate protecting film becomes polyfunctional, which results in reduction of parts.
With respect to the displaying quality of a liquid crystal display, compensation of viewing angle is improved. A viewing angle property of a liquid crystal cell can be improved by providing a discotic liquid crystal film on both sides of a twist nematic (TN) type liquid crystal cell (see for example, Japanese Patent O.P.I. Publication No. 7-191217).
With respect to improvement of viewing angle due to improvement of a liquid crystal mode, there is proposed a liquid crystal display employing a vertical alignment (VA) type liquid crystal cell in which a liquid crystal is substantially vertically oriented when voltage is not applied, and substantially horizontally oriented when voltage is applied (see for example, Japanese Patent O.P.I. Publication No. 2-176625.). The VA type liquid crystal cell provides wide viewing angle and high speed response as compared to a conventional TN type liquid crystal display, but requires further improvement as compared to CRT.
The VA type liquid crystal display provides black display by a liquid crystal layer approximately vertically oriented and a pair of polarizing plates each provided-in a crossed Nicol state on both sides of the liquid crystal layer. This display provides good black display observed from a direction normal to the display surface, but shows light leak observed from a direction (hereinafter referred to as an oblique viewing angle direction) inclined from the normal direction, resulting in poor black display.
This light leak in the oblique viewing angle direction results from the facts that birefringence occurs when the liquid crystal layer approximately vertically oriented is observed from the oblique viewing angle direction and that when the transmission axis of a pair of polarizing plates each provided in a crossed Nicol state on both sides of the liquid crystal layer is observed from the oblique viewing angle direction, the orthogonal relationship is deviated.
With respect to a method for compensating a viewing angle displaying quality from optical view points, there is disclosed in for example, Japanese Patent O.P.I. Publication No. 2003-262869 a designed value of a compensation film in an MVA type liquid crystal display, which is a multi-domain divided VA type liquid crystal display. However, this patent document does not disclose a liquid crystal display employing a polarizing plate with an optical compensation film prepared from a concrete resin nor a liquid crystal display employing a polarizing plate with a polarizing plate protecting film having an optical compensation property for viewing angle compensation. Recently, demand for a TV liquid crystal display for moving image is increased, and a liquid crystal display with a polarizing plate having improved displaying quality and high productivity is required also in this field.
A polarizing film of a polarizing plate is obtained by adsorbing iodine on the polymer film and stretching the resulting film. For example, a polyvinyl alcohol film is immersed in a solution called H ink containing a dichromatic substance (for example, iodine), and uniaxially stretched to orient the dichromatic substance in one direction.
As a protecting film of a polarizing plate, a cellulose resin or cellulose acetates are used, and of the cellulose acetates, cellulose triacetate is preferably used.
Generally, a polarizing plate protecting film made of cellulose resin is used for protecting physically a polarizing plate. As a manufacturing method of the film, a solution casting method is used which employs a solution containing a halogen-containing solvent. A solvent recovery cost is a big burden to this method. Various solvents other than the halogen-containing solvent have been examined, but solvents providing satisfactory solubility of the cellulose resin have not been found as alternative solvents. Instead of alternative solvents, a dissolution method such as a chilling method has been examined (see Japanese Patent O.P.I. Publication No. 10-95861.), but its industrialization is difficult and further study on it is necessary.
As a method for manufacturing a cellulose resin film as a polarizing plate protecting film, a solution casting method is carried out which casts on a support a solution in which cellulose resin is dissolved in a solvent and evaporates the solvents to dry. In the solution casting method, the solvents contained the cellulose resin web cast on the support need be removed by evaporation to dry. Accordingly, investment of equipment such as a drying device, a device for recovering evaporated solvents or a device for reproducing the recovered solvents is necessary, resulting in an increase in cost of manufacture. Reduction of cost of manufacture is a matter of great importance.
In view of the above, a process, which does not employ solvents to be evaporated for drying in film manufacture, is considered to overcome the above-described problems embraced in the solution casting method.
As the method to overcome the above-described problems, film formation according to a melt casting method is proposed (see Japanese Patent O.P.I. Publication No. 2000-352620.). Hitherto, manufacture of a polarizing plate protecting film according to the melt casting method is difficult and has not been put into practical use. It is necessary to solve problems in that film flatness is lowered due to contaminations at the lip of an extruder or film rupture is likely to occur at the slitting section during or after hot stretching.